Faulty Heat Exchanger Causes, Repair Costs & How To Fix It

James Elston
Written by James Elston
Updated on 4th December 2023
Posted on 3rd December 2023
Faulty Heat Exchanger Causes, Repair Costs & How To Fix It

Faulty Heat Exchanger On Boiler Plus How To Fix & Costs!

A faulty boiler heat exchanger is one of the most prevalent faults on most boilers; however, many people do not understand what a heat exchanger does, nor do they comprehend how to fix one.

In this article, you will discover all you need to understand about a heat exchanger, the faults they develop, and how to fix them.

If you don’t know what a heat exchanger is, you aren’t alone. Most people aren’t familiar with the term, much less what it does and how it can break down.

You should note that a faulty boiler heat exchanger is a common fault, regardless of the type and model of boiler you have.


What Is a Heat Exchanger?

A heat exchanger is a component within a boiler that transfers the energy your boiler creates from its supply of gas into hot water. Water goes through the boiler’s heat exchanger, where it heats up as it would in a kettle.

The hot water then passes through the boiler’s flow pipe and to the central heating system. It moves around your heating system, through the towel rails and radiators before going through the return pipe to the boiler.

Once it’s back in the boiler, the water goes through the heat exchanger again to repeat the entire process. You can see that the heat exchanger of any boiler is an integral part. If this part doesn’t function properly, the boiler will be unable to give hot water and central heating to a home.

In better boilers, replacing a faulty heat exchanger can be expensive so it may be better to find out the latest new boiler costs, especially if you have an older boiler.

Faulty Boiler Heat Exchanger: Common Issues

A boiler’s heat exchanger is an essential component and one that gets a lot of use, so it is no surprise that it frequently develops a fault.

When a faulty boiler heat exchanger happens, there are two causes or symptoms. They are:

  • Limescale build-up
  • Heating sludge build-up

Did you know? You must get your boiler serviced whilst it is under warranty. A boiler service costs around £100, and it could save you from hefty repair bills too.

Limescale build-up

boiler house with heat pump

Limescale builds up in any heating system; however, it is more likely to happen in areas with hard water. You can determine if your area has hard water by contacting your council. If your area has moderate or hard water, limescale could cause your faulty boiler heat exchanger problems.

Limescale comes from minerals found in all water; however, it is most common in hard water. The water flows around your heating system, causing the minerals to attach to the inside of your system’s pipes, towel rails, radiators, heat exchangers and other boiler components. Over time, the build-up creates a substance called limescale, which can cause the components to break down and stop working.


Limescale build-up can also result in leaks. These typically happen because the increased heat can lead to cracks in the heat exchanger. You can notice the leak as it comes out from the bottom of the boiler casing.


Limescale build-up can cause your boiler to go into lockdown. When this happens, the most common cause is the NTC thermistor. The NTC thermistor is the component that monitors the water temperature when it exits a boiler. If the cause of a hotspot is limescale and the water from the boiler is too hot, then the NTC thermistor recognises this, causing the boiler to lock out.


Limescale build-up in your heat exchanger can cause it to make noise. However, this noise comes as a whistling or kittling noise. This noise occurs because the limescale can create a hotspot on your heat exchanger, which happens when the water heats up too much, and you hear a bubbling noise caused by the water heating up inside the faulty boiler heat exchanger.

Ways to fix limescale build-up

Limescale build-up is a problem that requires the assistance of a qualified professional. You should note that the key to handling this problem of limescale build-up is to catch it early. A professional boiler engineer uses a silencer to break down limescale in the heating system. A silencer like the Sentinel X200 acts as a cleaner to remove any limescale buildup.

The Gas Safe registered engineer might also add a limescale reducer to your boiler. It can collect limescale floating around the central heating system to ensure a build-up doesn’t occur.

If your faulty boiler heat exchanger has a severe limescale build-up, it might be necessary to change the component if the engineer cannot clean it. You should note that getting a new one to replace your faulty boiler heat exchanger is not cheap. You can expect to pay about £500 or more for a new one.

Heating sludge build-up

This type of sludge is a common issue with most heating systems; however, it is most prevalent in older heating systems. This sludge happens when rusty metal or dirt breaks away from the pipes, joints, or radiators and ends up in the water that goes around the system. Over a period, this breakaway debris can form into sludge in various parts of the system, especially in the boiler.

If the build-up is large enough in certain areas of the system, it can create a blockage. It can also stop the boiler components from functioning properly.

What are the symptoms of heating sludge build-up in a heat exchanger?


A lockout is a mechanism triggered when the boiler senses it has a fault and the PCB will tell the boiler to shut down Sludge build-up in the heat exchanger can often result in a lockout. The lockout tends to happen when the water is overheating; however, it can also result from a water circulation problem.

Debris and sludge can restrict water flow along with the heat exchanger to the point where a boiler lockout is triggered. If this is a problem your boiler has, you can often determine it by registering the difference in temperature between the return pipe and the flow. Generally, the flow pipe is warm, while the return pipe is a little colder when everything is functioning effectively.

However, if your boiler has a blockage in its heat exchanger, the return pipe is likely to be a lot colder than it should be. Another method of checking for this fault is to bleed the radiators. Once you release some air, you should have water come out. If the water that comes out is black, you have confirmation that there is a build-up of sludge within your central heating system.


A boiler with severe sludge build-up is likely to be quite noisy when in operation. When coming from the heat exchanger, this noise means pieces of debris or sludge are passing through the heat exchanger and scraping against it. This movement results in a tapping or scratching sound when the boiler is in use.

How to fix sludge build-up

Central heating sludge can negatively affect your boiler’s performance; however, it isn’t typically as severe as limescale. Nevertheless, it has to be fixed, and a way to do that is to hire a qualified boiler engineer to carry out the work. This certified individual is trained to inspect the heat exchanger and remove any sign of sludge or debris from it. The heating engineer will also run a power flush or hot flush to clear debris from the rest of your heating system.

However, you should note that a power flush is extremely powerful and can damage older heating systems. When the flush is completed, the Gas Safe engineer will place an inhibitor into the central heating system. The inhibitor helps to clear things out, which prevents the build-up of sludge in the future.

You can ask the heating engineer to retrofit your boiler with a magnetic system filter. The filter’s job is to collect any metal debris that floats in your heating system, preventing the build-up of sludge in the future. You should note that this filter will have to be intermittently cleaned; however, this is something you can do in conjunction with your yearly boiler service.

Things like this can prevent other problems such as a faulty diverter valve as sludge causes more blockages, We always suggest cleaning your central heating system before installation of a new boiler.

Replace Or Repair A Broken Heat Exchanger?

Fortunately, if your boiler is still under warranty, this cost is covered, and all you have to do is pay labour costs. Labour costs aren’t as expensive as getting a new boiler heat exchanger; however, it can depend on where you live. If your boiler isn’t under warranty, you will have to shoulder the costs yourself.

Moreover, if you have an older boiler, you can consider if replacing the boiler is less expensive than trying to fix it. I

f the Gas Safe engineer cannot fix the faulty boiler heat exchange, you will have no choice but to buy a new component or a new boiler. While a new boiler might cost slightly more, the price difference isn’t as much as one might believe. Furthermore, a new boiler can help you save money in the long run.

Older boilers are bound to develop faults the more you use them, and you are likely to replace major components. When you invest in a new boiler, you choose to have a new warranty, which means that all your repairs are covered for that duration. Moreover, modern boilers are more energy-efficient than older ones, so you will likely lower your energy costs.

If you are thinking about getting a boiler replacement, you will need many quotes. With that in mind, head here to get new boiler quotes in just 20 seconds without having to leave your home.

Boiler Central aims to get you the very best boiler prices and cheapest boiler upgrades at all times. With this service, you can be sure that you are getting a wide range of boilers to select from, should you be interested in making a change. This isn’t something any other boiler quotation service can say.

Faulty Heat Exchangers Conclusion

A heat exchanger is an essential part of a boiler, so important that the boiler doesn’t work when it goes wrong. When you have a faulty boiler heat exchanger, it could be because of a build-up of limescale or sludge.

Reading the above guide, you should be able to determine the cause of your faulty boiler heat exchanger and the steps you need to take to get your boiler up and working again. Always remember that it is better to have an experienced and licensed boiler repair engineer handle the work on your boiler.

Written by
James Elston
James Elston Director Of Boiler Central
Posted on: 3rd December 2023

James Elston is our resident boiler replacement and heating expert here at Boiler Central. With over 20 years experience in the boiler installation industry, James ensures that he knows everything there is about our Gas Safe boiler installations, energy saving and home heating solutions. This can be from simply procuring the latest best combi boilers, to reviewing and ensuring that Boiler Central maintains the highest standards across our boiler installation company.

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